Gas-Surface Interaction Physisorption Chemisorption Part of MATERIALS SCIENCE
Gas-Surface Interaction Physisorption Chemisorption Part of MATERIALS SCIENCE & AALearners LearnersGuide Guide ENGINEERING AN INTRODUCTORY E-BOOK
Advanced Reading Anandh Subramaniam & Kantesh Balani Materials Science and Engineering (MSE) Indian Institute of Technology, Kanpur- 208016 Email: [email protected], URL: home.iitk.ac.in/~anandh http://home.iitk.ac.in/~anandh/E-book.htm Reactions at Solid Surfaces Gerhard Ertl Wiley, Hoboken, New Jersey (2009)
Gas-Surface Interaction: an Overview When gas molecules impinge on a surface, they transfer momentum and give rise to pressure on the surface. Some of the molecules may stick on the surface and may be weakly bonded to the surface (by van der Waals forces) called Physisorption. (E.g. Ar on Pd surface) If the interaction is stronger and it is referred to as Chemisorption. (E.g. CO on Ni(111)). During chemisorption the entities of the molecule may break up called Dissociative Chemisorption. (E.g. H2 on Pd surface). Coverage If adsorption is in equilibrium with desorption, at constant coverage () energy of the
adsorption (Ead) is given by the Clausius-Clapeyron equation. Thus, a plot of lnp with 1/T at constant coverage () will give us the isosteric heat of adsorption (at that coverage). Fig.1 shows the Ead versus coverage for CO adsorbed on Pd(111). The adsorption energy remains constant up to ~ 0.33 and then drops by 2 kcal/mol due to a change in the adsorption geometry, which arises due to repulsion between the adsorbed molecules. d (ln p ) E ad d (1 / T ) R
CO adsorbed on Pd(111) Interaction between adsorbate molecules The interactions between adsorbate molecules may be repulsive or attractive. Repulsive interactions can arise in two ways: (i) Direct interaction or (ii) that mediated by the substrate. The direct interaction can arise from dipoledipole interaction or from orbital overlap. The indirect nature of the interaction is mediated through the electronic structure of the substrate. The attractive interactions are usually mediated by the substrate. Fig.1 shows the interaction potential between two oxygen atoms adsorbed on Ru (0001) surface, as a function of their separation (measured in terms of the lattice constant of the substrate surface).
Chemisorption The Lennard-Jones diagram (Fig.1) helps us understand dissociative adsoption. If a
diatomic molecule A2 approaches a surface, it will first experience (weak) bonding as A2,ad. Dissociation of the free molecule would require the dissociation energy Ediss, and the two atoms would then form strong bonds with the surface (Aad). The crossing point of the two lines marks the activation energy for dissociative adsorption and determines the kinetics of adsorption (see below), while the adsorption energy Ead for A2 ! 2Aad is related to the surfaceadsorbatebondenergyES-A throughES-A 12 Ead Ediss. Chemisorption energy and surface diffusion Table-1 lists the chemisorption energy of CO molecule on some metallic surfaces. The dissociation energy for the corresponding carbonyl compounds are also given in the table.
The dissociation energy for the carbonyl compounds larger by ~3050%. This seems to be the case for both mono- or multinuclear compounds. The surface chemisorption energy is lower as the valency (no of unsaturated bonds) of the surface atoms is lower (as compared to a free atom). This also implies that the chemisorption energy should be higher on crystallographic planes with a lower density. Surface steps and kinks offer preferred sites for adsorption with higher sorption energy. The difference in the energy between different sites on the surface, determines the activation energy for surface diffusion. This activation energy is about 20% of the sorption energy and hence given sufficient kinetic activation the molecule will diffuse on the surface before desorption.
Koponya - Cranium Az arckoponya csontjai Humánbiológia gyakorlat 4.2. Az arckoponya csontjai 1. Rostacsont - Os ethmoidale A két szemüreg közét elfoglaló, egészben kocka alakú, papírvékony csontlemezekből álló, légtartalmú üregekkel rendelkező csont függőleges lemeze - lamina perpendicularis (a csontos orrsövény...
Chapter 1 Notes "Human Beginnings" ... as containers Crops grown People began settling into communities Earliest villages = Jericho (in modern West Bank) and Catal Huyuk (in present-day Turkey) Agricultural Revolution Invented plow and trained oxen to pull it Used...
Objectives. Identify common challenges of implementing alcohol and other drug education within Fraternity and Sorority Life. Understand how a harm reduction program or harm reduction principles can be implemented with Fraternity and Sorority Life.
CS5412 Spring 2014 (Cloud Computing: Birman) Long list on our course web page Some really good ones from the smart power grid, but flexibility to do something less ambitious or even to propose a project of your own
Den litterære revolution De nye litterære retninger A. Kritisk realisme (Henrik Pontoppidan) B. Naturalisme (J.P.Jacobsen) C. Impressionisme (Herman Bang) Kritisk realisme Henrik Pontoppidan Mennesket som produkt af samfundsmæssige processer: Præcis skildring af samtiden bygget på iagttagelse, psykologisk indsigt, viden om...
A "New World" One Italian sailor, Christopher Columbus, believed that he could reach Asia by sailing west across the Atlantic. The king and queen of Spain gave Columbus the money to make his journey. In October 1492, Columbus landed on...
Ready to download the document? Go ahead and hit continue!